1. Field of the Invention
The present invention relates to a mobile station apparatus used in a digital cellular radio system such as a mobile telephone system or cordless telephone system and, more particularly, to a mobile station apparatus using the code division multiple access (CDMA) scheme as a radio channel access scheme between the apparatus and a base station and a spreading code synchronization method used for the apparatus.
2. Description of the Related Art
As a communication scheme applied to a mobile communication system, a spread spectrum communication scheme which is resistant to interference and disturbance has recently received attention. The spread spectrum communication scheme is mainly used to realize a cellular radio system using the CDMA scheme.
According to a cellular radio system using the CDMA scheme, for example, in the apparatus on the transmission side, digital speech data or image data is modulated by a digital modulation scheme such as the PSK modulation scheme, and the modulated transmission data is converted into a wide-band baseband signal by using a spreading code such as a pseudorandom noise code (PN code). Thereafter, this signal is up-converted into a signal in a radio spectrum to be transmitted. In the apparatus on the reception side, the received signal in the radio spectrum is down-converted into a signal having an intermediate frequency or baseband frequency, and the signal is de-spread by using a spreading code identical to the spreading code used in the apparatus on the transmission side. Thereafter, digital demodulation of the signal is performed by a digital demodulation scheme such as the PSK demodulation scheme to reproduce the received data.
That is, in the CDMA scheme, different spreading codes are assigned to radio communications between a plurality of mobile station apparatuses and base stations to ensure channel separation between the respective radio communications.
In a system of this type, when a mobile station apparatus starts communication, synchronization must be established with respect to the spreading code of a radio signal transmitted from a base station. As a synchronization scheme for this purpose, the following scheme has been proposed.
For example, a PN code has a length of 215 chips, and a plurality of base stations generate PN codes from different preset chip phases. A chip phase from which generation of a PN code is started is called an offset.
When the power supply of a given mobile station apparatus is turned on, a PN code generator generates PN codes while gradually shifting the chip phase from zero offset. The apparatus monitors the correlation values between these PN codes and the PN codes of indicating signals broadcasted from base stations so as to search the offsets of the PN codes used in the base stations. Each time an offset of a PN code used in a base station is captured by this search, the apparatus receives a sync channel to check system identification information (SID) for identifying the mobile communication network. If this SID corresponds to a desired network, the apparatus shifts to a standby state. If a communication request is generated in this standby state, the apparatus captures a signal, of radio signals transmitted from a plurality of base stations in the desired network, which has the highest reception level. Thereafter, the apparatus shifts to a communication state while maintaining the PN code synchronization between the apparatus and the base station as a transmission source for this radio signal.
If the SID does not correspond to a desired network, the apparatus generates PN codes while shifting the chip phase from the last captured offset. With this operation, the apparatus searches the offsets of the PN codes of indicating signals broadcasted from other base stations. Each time an offset of a PN code is captured, a sync channel is received to check an SID. If this SID corresponds to a desired network, the apparatus shifts to the standby state. If the SID does not correspond to a desired network, the apparatus searches the offsets of PN codes again.
Subsequently, a search the offsets of PN codes and determination of SIDs are repeated until the SID of the desired network is received.
The following problems to be solved are posed in the conventional proposed PN code synchronization scheme. In the metropolitan areas, a plurality of communication companies (carriers) may install base stations in a common service area to constitute cells, thereby constructing a cellular radio system. In this case, if mobile communication networks of a plurality of carriers respectively use different radio frequencies, no problem is posed. In some case, however, such networks share radio frequencies for the sake of the effective use of the radio frequencies. In this case, the radio channels used between the carriers must be discriminated from each other by using other means. According to a conventional discrimination scheme, for example, different offsets are set for PN codes assigned to base stations in the mobile communication networks of the respective carriers.
In such a system, however, when PN code synchronization is to be established by using the above conventional PN code synchronization scheme, a given mobile station apparatus may continuously capture only offsets assigned to the networks of undesired carriers depending on the assignment of the offsets to the networks of the respective carriers. As a result, it may take a long period of time to capture a base station of a desired carrier.
Assume that carriers X and Y have base stations BSx1 to BSx3 and BSy1 to BSy3 installed in the respective common service areas to form cells Ex1 to Ex3 and Ey1 to Ey3, offsets "10", "15", and "25" are respectively assigned to the base stations BSx1 to BSx3, and offsets "40", "45", and "50" are respectively assigned to the base stations BSy1 to BSy3, as shown in FIG. 6.
When a mobile station apparatus PS at a position where it can receive indicating signals from all the base stations BSx1 to BSx3 and BSy1 to BSy3 is to establish PN code synchronization with respect to one of the base stations BSy1 to BSy3 of the desired carrier Y, the following synchronization establishing operation is performed. The mobile station apparatus PS starts to search a PN code from the offset "0", and sequentially captures the offsets "10", "15", and "25" of the base stations BSx1 to BSx3 of the undesired carrier X. At the fourth search, the mobile station apparatus PS captures the offset "40" of the base station BSy1 of the desired carrier Y.
Before capturing the offset "40" of the base station BSy1 of the desired carrier Y, the mobile station apparatus PS must capture all the offsets "10", "15", and "25" of the base stations BSx1 to BSx3 of the undesired carrier X. In addition, the mobile station apparatus PS must receive an SID through a sync channel and perform determination processing every time an offset is captured. For this reason, it takes a long period of time to capture a base station of the desired carrier Y.